Competitive Intermolecular Pericyclic Reactions of Free and Complexed Cyclopentyne

Abstract

Intermolecular competition for cyclopentyne by different alkenes supports the hypothesis that organolithium-promoted decomposition of precursors to cyclopentyne affords one or more lithium-ion-complexed species. Competition reactions with mixtures of 2,3-dihydropyran do not clearly differentiate between complexed and unencumbered (free) forms of cyclopentyne, but those involving spirodiene 2 and cyclohexene do. Remarkably, the cycloaddition reactions of free cyclopentyne are not diffusion-controlled despite its high reactivity.

π‐Strain‐Induced Electrophilicity in Small Cycloalkynes: A DFT Analysis of the Polar Cycloaddition of Cyclopentyne towards Enol Ethers

“…Therefore, the activation enthalpy for the polar [2+2] cycloaddition between 1 and 16 is 3.6 kcal mol –1 lower in energy than that associated with the [1+2] cycloaddition between 1 and ethylene. This result is in agreement with the experimentally observed chemoselectivity in the [2+2] cycloaddition of 1 to 5 and 6 (see Scheme and ref 7…”

“…In the absence of these species, carbene or biradical mechanisms can be operative. This result is in agreement with the chemoselective [2+2] cycloadditions of cyclopentyne to the electron‐rich dihydropyran and cyclohexene 7…”

“…In order to test this prognosis we studied the polar [2+2] cycloaddition between cyclopentyne ( 1 ) and methyl vinyl ether ( 16 , see Scheme ) as a model for the [2+2] cycloaddition between 1 and dihydropyran ( 5 , see Scheme ) 7. Recently, we have shown that the regioselectivity in the [2+2] cycloaddition of the benzyne 9 to ketene acetals can be correctly explained by a polar process characterized by the nucleophilic attack of the ketene acetal 14 to the substituted benzyne 13 (see Scheme ) 25.…”

“…However, a more complex product distribution was obtained when 4 was allowed to react with dihydropyran ( 5 ), a result probably associated with the norbornane chemistry (see Scheme ). Recently, Gilbert and Hou have studied the chemoselectivity of the [2+2] cycloaddition of cyclopentyne 1 to dihydropyran ( 5 ) and cyclohexene ( 6 ; see Scheme ) 7. The ratio 3.3:1 ( 7 : 8 ) suggests a preference for the reaction of 1 with the electron‐rich alkene 5 .…”

12 Reaction mechanisms : Part (iii) Pericyclic reactions

“…The use of cyclopentyne in [212] and [214] cycloaddition reactions was studied by Gilbert and Hou. 24 One important question in the study is the mechanistic importance of a cyclopentyne-lithium bromide complex that can form during the synthesis of the alkyne, depending on the precursor. In a competitive [212] cycloaddition with cyclohexene versus 2,3-dihydropyran, the cyclopentyne precursor is not important in determination of the product ratio.…”